Rifamycin derivatives bearing a heterocyclic ring which is condensed at the 3,4-positions are known from the art literature, including the patent literature. As an example, in South African Pat. No. 68/0903 there are claimed pyrrolo [5,4-c] rifamycin SV derivatives, whereas German Patent Publications Nos. 2,739,671 and 2,739,623 describe some imidazo [5,4-c] rifamycin SV compounds which bear substituent at the positions 1 and 2. Thiazolo [5,4-c] rifamycin SV (rifamycin P) derivatives are reported in the German Patent Publication No. 2,741,066.